Thin insulating films of semiconductor devices may include a gate insulating film such as SiO.sub.2 film, a capacitor insulating film such as Si.sub.3 N.sub.4 film, a layer-insulating film such as PSG (phosphorus-silicon glass) film and BPSG (boron-phosphorus-silicon glass), etc. As the CVD material for forming these thin films, gaseous materials such as SiH.sub.4, NH.sub.3, PH.sub.3 and B.sub.2 H.sub.6 have heretofore been used.
The recent trend of making an electronic device into a three-dimensional structure and forming circuits on increased number of layers of a laminated structure requires a thin insulating film of even more improved flatness. In this circumstance, an alkoxide has come to be used as a liquid CVD material due to its ability of forming a high-quality thin film with little defect such as void. For example, tetraethoxysilicon (Si(OC.sub.2 H.sub.5).sub.4) is practically used as a CVD material for forming SiO.sub.2 film and trimethoxyboron (B(OCH3).sub.3), trimethoxyphosphorus (P(OCH.sub.3).sub.3), etc. are practically used as a CVD material or forming BPSG film.
In addition, Ta.sub.2 O.sub.5 film, HfO.sub.2 film, etc. having a dielectric constant several times as high as that of SiO.sub.2 film have been recently developed. The Ta.sub.2 O.sub.5 film has been used as the capacitor insulating film. As the CVD material for these thin films, used are pentaethoxytantalum (Ta(OC.sub.2 H.sub.5).sub.5) and tetra-t-butoxyhafnium (Hf(OC(CH.sub.3).sub.3).sub.4).
An extremely high purity is required for the liquid CVD materials such as the alkoxide to form a high-quality thin insulating film. Therefore, impurities such as alcohols, polymers of alkoxides and water should be removed from the alkoxide by distillation before being used as the CVD materiel.
However, when a liquid alkoxide distilled by a conventional process is used as the liquid CVD material, a thin insulating film having a sufficiently high quality cannot be obtained although defects such as voids in the deposited thin insulating film decreases in comparison with those in a thin insulating film deposited by using a gaseous CVD material. This failure in obtaining a sufficiently high quality is assumed that the vapor phase deposition is adversely affected by the volatile impurities such as halogens, hydrogen halides, oxygen and carbon dioxide present in the alkoxide and the polymers and degradation products of the alkoxide formed by heating for vaporization. However, no method has been reported in successfully eliminating the above problem.